In order to record an input sound properly, a limiter for the input sound is an important function. A normal limiter operates with respect to all frequency bands of the sound to be recorded. Thus, for example, even when a peak is detected at a frequency band different from the frequency band of the sound desired to be recorded, gain reduction is executed for all frequency bands. Because of this, sound in the frequency band of the sound it is desired to record is also reduced.
As a countermeasure in such a case, a multi-band limiter is being proposed. In a multi-band limiter, the whole frequency band is divided into a plurality of bands, and a limit process is executed independently for each frequency band, so that the gain reduction due to an instantaneous peak in a certain frequency band does not affect other frequency bands.
WO 2011/048741 discloses a multi-band compressor, and describes that, in a typical multi-band compressor of the related art, compressors of the frequency bands operate independently from each other, and that, in order to solve the problem associated with the independent operation, a gain value is calculated from a signal level calculated in each frequency band, and the gain value used for a compression process is limited by comparing the gain values calculated at various frequency bands.
One problem associated with the multi-band limiter is a slope of a filter when the frequency band is divided. Filters for cutting out adjacent frequency bands are normally set to cross at −6 dB (hereinafter referred to as “cross-over filter”), so that the frequency characteristic becomes flat after the divided frequency bands are combined.
However, in the filter process with such a cross-over filter, a band in which the filter is not applied, which is also called a dead zone, occurs in frequency bands above and below the crossed frequency. Specifically, in this zone, originally, even for a signal having a level exceeding a threshold, and for which the gain should be reduced because the level of the input sound is attenuated by the filter, the signal level after the attenuation becomes less than or equal to the threshold and the gain cannot be reduced. In addition, when the slope of the filter is not steep, signals of the same frequency would exist in both adjacent frequency bands, and in this case, the limiter is applied to one signal but not to the other. Consequently, when the signals are added in the combining process, a frequency band in which the limiter is not applied is generated, and the dead zone may be further widened.
FIGS. 4A and 4B show a frequency characteristic of a multi-band limiter of the related art. The frequency bands are divided into three bands, LOW, MIDDLE, and HIGH, the signals are divided with the filters of these bands crossing at −6 dB (FIG. 4A), the limit process is executed for each band (FIG. 4B), and the signals of the bands are added and combined. In other words, the flow is:    frequency band division (−6 dB cross)→limit process for each band→combining.
However, because the signal is attenuated in the frequency band near adjacent frequency bands as shown in FIG. 4A, a signal which should be limited by the threshold, that is, the limiter level, is not limited, and the limiter is not applied. FIG. 4B shows the dead zone of the limiter thus created. When the dead zone of the filter appears, if there is a peak level of the sound volume in this portion, the peak level cannot be limited, and sound recording failure may result due to overpeak of the sound volume.